Boolean modifier not working with hiccup
Booleans are odd things and need certain things to be right before they work, one of the things that stops them working is Scale.

Make sure you have the scale of both objects set to $1,1,1$. by pressing Ctrl + A to Apply Scale (adjust geometry to match the scaled version and set scale to $1$.

Also try and make sure your models are fully closed (solid with no holes). Since  your working with  a scan of a face, try to add geometry on the back, or use the Solidify modifier with a large thickness, to turn it into a solid object.

I also find the Bool Tools addon helps me a lot as well.

Occasionally I have also used a work around of copying the Boolean objects and pasting them into a fresh scene and voila, it works! Not always a reason given, you can then paste them back to where they need to be.